The cleaning of semiconductor wafers is often a critical step in the fabrication processes used to manufacture integrated circuits or the like. The geometries on wafers are often on the order of fractions of a micron, while the film thicknesses may be on the order of 20 Angstroms. This renders the devices highly susceptible to performance degradation due to organic, particulates or metallic/ionic contamination. Even silicon dioxide, which is used in the fabrication structure, can be considered a contaminant if the quality or thickness of the oxide does not meet design parameters.
Although wafer cleaning has a long history, the era of “modern” cleaning techniques is generally considered to have begun in the early 1970s when RCA developed a cleaning sequence to address the various types of contamination. Although others developed the same or similar processes in the same time frame, the general cleaning sequence in its final form is basically the same.
The first step of the RCA cleaning sequence involves removal of organic contamination using sulfuric acid and hydrogen peroxide mixtures. Ratios are typically in the range of 2:1 to 20:1, with temperatures in the range of 90-140 degrees Celsius. This mixture is commonly called “piranha.” A recent enhancement to the removal of organic contamination replaces the hydrogen peroxide with ozone that is bubbled or injected into the sulfuric acid line.
The second step of the process involves removal of oxide films with water and HF (49%) in ratios of 200:1 to 10:1, usually at ambient temperatures. This processing typically leaves regions of the wafer in a hydrophobic condition.
The next step of the process involves the removal of particles and the re-oxidation of hydrophobic silicon surfaces using a mixture of water, hydrogen peroxide, and ammonium hydroxide, usually at a temperature of about 60-70 degrees Celsius. Historically, ratios of these components have been on the order of 5:1:1. In recent years, that ratio has more commonly become 5:1:0.25, or even more dilute. This mixture is commonly called “SC1” (standard clean 1) or RCA1. Alternatively, it is also known as HUANG1. Although this portion of the process does an outstanding job of removing particles by simultaneously growing and etching away a silicon dioxide film on the surface of a bare silicon wafer (in conjunction with creating a zeta potential which favors particle removal), it has the drawback of causing metals, such as iron and aluminum, in solution to deposit on the silicon surface.
In the last portion of the process, metals are removed with a mixture of water, hydrogen peroxide, and hydrochloric acid. The removal is usually accomplished at around 60-70 degrees Celsius. Historically, ratios have been on the order of 5:1:1, but recent developments have shown that more dilute chemistries are also effective, including dilute mixtures of water and HCl. This mixture is commonly referred to as “SC2” (standard clean 2), RCA2, or HUANG2.
The foregoing steps are often run in sequence, constituting what is called a “pre-diffusion clean.” Such a pre-diffusion clean insures that wafers are in a highly clean state prior to thermal operations which might incorporate impurities into the device layer or cause them to diffuse in such a manner as to render the device useless. Although this four-step cleaning process is considered to be the standard cleaning process in the semiconductor industry, there are many variations of the process that use the same sub-components. For example, the piranha solution may be dropped from the process, resulting in a processing sequence of: HF−>SC1−>SC2. In recent years, thin oxides have been cause for concern in device performance, so “hydrochloric acid last” chemistries have been developed. In such instances, one or more of the above-noted cleaning steps are employed with the final clean including hydrochloric acid in order to remove the silicon backside from the wafer surface.
The manner in which a specific chemistry is applied to the wafers can be as important as the actual chemistry employed. For example, HF immersion processes on bare silicon wafers can be configured to be particle neutral. HF spraying on bare silicon wafers typically shows particle additions of a few hundred or more for particles at 0.2 microns nominal diameter.
Although the four-chemistry clean process described above has been effective for a number of years, it nevertheless has certain deficiencies. Such deficiencies include the high cost of chemicals, the lengthy process time required to get wafers through the various cleaning steps, high consumption of water due to the need for extensive rinsing between chemical steps, and high disposal costs. The result has been an effort to devise alternative cleaning processes that yield results as good as or better than the existing four-chemistry clean process, but which are more economically attractive.
Various chemical processes have been developed in an attempt to replace the existing four-chemistry process. However, such cleaning processes have failed to fully address all of the major cleaning concerns of the semiconductor processing industry. More particularly, they have failed to fully address the problem of minimizing contamination from one or more of the following contaminants: organics, particles, metals/ions, and silicon dioxide.
Accordingly, there is a need for improved systems and methods for processing and cleaning wafers or workpieces.